1. Field of the Invention
The present invention relates to an extreme ultraviolet lithography (EUVL) process, and more particularly, to a method of correcting deviations of critical dimensions of patterns formed on a wafer during an EUVL process.
2. Description of Related Art
In the fabrication of semiconductor devices, as the design rule is reduced to increase integration density, a lithography process for obtaining higher resolution is required. By a deep ultraviolet lithography (DUVL) process, a pattern having a dimension of about 250 nm can be formed using a 248-nm KrF laser as a light source. Also, when a 193-nm ArF laser is used as a light source, a pattern having a dimension of about 100 to 130 nm can be formed. However, the above-described DUVL process has a technical limitation in obtaining sub-100-nm resolution. For this reason, there have been intensive studies on an EUVL process that makes use of extreme ultraviolet rays (which are called soft X-rays) as a light source.
The EUVL process is notably different from the DUVL process in that the extreme ultraviolet rays used as a light source have a wavelength of 13.4 nm in which most materials have high light-absorbing properties. Accordingly, unlike in the DUVL process, a transmission photo mask cannot be used. Instead, a reflection photo mask should be used in the EUVL process. As disclosed in U.S. Pat. No. 5,052,033, the reflection photo mask basically includes a mask substrate, a reflection layer disposed on the mask substrate, and absorption patterns disposed on the reflection layer.
Meanwhile, in a lithography process, the uniformity of critical dimensions of patterns formed on a wafer is degraded with a reduction in the design rule. That is, when the patterns that are designed in the same dimension are transferred onto the wafer through the same photo mask, the dimensions of the patterns are undesirably changed during an exposure process depending on the positions to where the patterns are transferred. With recent reductions in the design rule, the change in the dimensions of the patterns is getting more serious. The uniformity of critical dimensions of the patterns formed on the wafer is affected by a variety of factors, such as a photoresist coating process, a baking process, an exposure apparatus, a photo mask, a developing process, and an etching process. In particular, shot uniformity, which refers to a dimension or a difference between the dimensions measured on a wafer after an exposure process, considerably depends on an exposure apparatus and a photo mask.
Nowadays, the DUVL process makes several attempts to improve the uniformity of critical dimensions of patterns formed on a wafer. Specifically, a diffraction grating or filter is provided on a rear surface of a photo mask to account for a change in the illumination system used for an exposure process. Alternatively, a method of preparing a filter on a photo mask to vary the energy of a laser pulse used as an exposure source is employed. However, these methods are applicable to the DUVL process that makes use of the transmission photo mask, but cannot be applied to the EUVL process that makes use of the reflection photo mask. Therefore, it is necessary to develop a new method for improving the uniformity of critical dimensions of patterns formed on a wafer, and to increase the yield of the reflection photo mask, which is more expensive than the transmission photo mask, during the EUVL process.